A concentrating solar power unit may operate to concentrate incoming light onto a solar cell. It may be desirable to couple an optical element to the solar cell in order to increase an acceptance angle of the concentrating solar power unit, to homogenize the light source over the surface of the cell, and/or to further focus the light. An upper (i.e., incoming) surface of the optical element should be retained in a particular spatial position relative to other optical elements in the system, and a lower (i.e., outgoing) surface of the optical element should be retained in a particular spatial position relative to an active area of the solar cell. The size and weight of the optical element typically prohibit bonding the optical element directly to the fragile surface of the active area as a means of achieving this positioning.
Some conventional concentrating solar power units use a three part mounting scheme to retain an optical element on a solar cell. The solar cell is mounted to a back side of a substrate, and a lower holder is mounted to a front side of the substrate. Both the lower holder and the substrate define openings to allow incoming light to pass to the solar cell. The openings are filled with a PDMS (silicone) gel and an optical element is placed in the openings such that the lower holder positions the bottom of the optical element over cell. The gel is cured between the bottom of the optical element and the solar cell to a thickness greater than the final operating thickness.
An upper holder is then placed to position the top of the optical element over the cell and to exert some compressive force against the cell. Placement of the upper holder therefore also serves to compress the gel. This compression may push out bubbles that may have formed during the cure and fixes a gap between the optical element and the cell.
The upper holder may include tabs to locate the top surface of the optical element. These tabs may plastically deform over time and reduce the compression of the gel. Moreover, although many packaging houses are familiar with silicone dispense, the size and geometry of the above-described assembly may create difficulties in high volume packaging lines.
Overcompression may cause the optical element to damage the cell. The gel itself may flow out of the gap over time (e.g., due to thermal pumping, etc), thereby degrading the optical coupling. The lower holder may be subject to oxidation if it receives highly concentrated light during operation. Products of the oxidation may be absorbed into the optical coupling or may be deposited on the optical element, degrading the optical performance and possibly leading to failure of the power unit.
Improved systems to retain an optical element on a solar cell are desired. Such systems may improve manufacturability, the retention of the optical element, the maintenance of the optical coupling and/or the quality of an optical coupling.